RFR: 8253146: C2: Purge unused MachCallNode::_arg_size field

[Aleksey Shipilev]

While doing changes in related code, noticed that `MachCallNode::_arg_size` is computed needlessly, taking memory
without a good reason. Let's purge it.

Testing: text search for _argsize; Linux x86_64 {release,fastdebug,slowdebug} builds; Linux x86_64 tier1; Linux ppc64
fastdebug build

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Commit messages:
 - 8253146: C2: Purge unused MachCallNode::_arg_size field

Changes: https://git.openjdk.java.net/jdk/pull/167/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=167&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253146
  Stats: 9 lines in 3 files changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/167.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/167/head:pull/167

PR: https://git.openjdk.java.net/jdk/pull/167